Entries in Fukushima 50
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ABC News(TOKYO) -- They are the nameless brave men who are working as the last line of defense at Japan's crippled Fukushima nuclear plants. They stayed behind while everyone else was sent nearly 15 miles away and radiation soars to menacing levels.

There are 200 of them and they work in shifts of 50, earning the nickname the Fukushima 50.

At one point, even these men were pulled back 500 yards from the deteriorating nuke plants, but by Wednesday it appeared the crisis team was heading back in.

Japanese Prime Minister Naota Kan told the volunteers, "You are the only ones who can resolve a crisis. Retreat is unthinkable," according to the Financial Times.

They are working as temperatures at the plants soar to nerve-wracking levels, radiation is leaking, rain may be carrying it down upon them, and a toxic fire burns, likely spewing more radiation into the atmosphere.

There is little information about who actually stayed behind, but nuclear experts say the skeleton crew is likely made up not of managers, but of technicians, men who have the schematics of the plant in their head and can fix pipes and unclog vents.

They've gone into battle, crawling at times through dark mazes, armed with flashlights and radiation detectors, wearing full body hazmat suits and breathing through cumbersome oxygen tanks.

Potentially deadly doses of radiation surround them as they work, and their suits do little to prevent radiation from seeping into their bodies.

Radiation sickness sets in after exposure to 1,000 milliseiverts (mSv) of radiation at once.

It's not clear what doses of radiation these men are absorbing, but American and Japanese regulations on radiation exposure are similar: both countries have a total dose of 50 mSv that workers are allowed to be exposed to in one year. But in an emergency situation, workers are allowed to exceed that value.

According to the NHK, the Japanese raised the maximum dose allowed to 100 mSv for the Fukushima 50, and on Tuesday raised the number to 500 mSv -- the international value for the maximum allowed dose in a state of emergency.